This invention relates to the detection of coded or modulated electrical currents that are transmitted through the rails of a railroad track for control purposes and, more particularly, to an improved inductive sensor which suppresses high level noise that would otherwise interfere with detection of the control information.
Railroad signalling has traditionally been based upon the concept of protecting zones of track, called "blocks," by means of some form of signal system that conveys information to the locomotive engineer about the status of the track ahead. Typically, wayside signal lights are located along the track and are controlled by electrical logic circuits responsive to the presence of trains and the status of blocks that are relevant to a given wayside signal. Each wayside signal is thus caused to display a pattern of lights, called the "aspect" of the signal, which is visible to the engineer in the locomotive and indicates the status at that location.
A more advanced signalling system in widespread use is referred to as cab signalling and may be used with or without wayside signal lights. In cab signalling the same logic that determines block status for display on the wayside signals is also used to generate one of several forms of encoded electrical current in the rails, block status being represented by the selection of the code rate used. Inductive pickup coils are mounted on the locomotive ahead of the lead wheels and just above the rails for the purpose of sensing the magnetic fields around the rails produced by the encoded current. In modern systems a computer on board the locomotive decodes the detected information to determine the status and indicates the proper aspect in the engine cab by a speed limit value display or a pattern of lights in the same manner as a wayside signal. One advantage, of course, is that the information is made available to the train crew on a continuous basis and updated immediately when changes in status occur, rather than restricting the communication of status information to periodic intervals along the track at which the engineer is required to observe and read the next wayside signal.
The pickup coils typically used in cab signal systems are iron core inductors employed in pairs, one being mounted above each rail. The carrier frequency of the coded cab current for freight operations is typically in the range of from 40 Hz to 100 Hz but may be as high as 250 Hz. Examples of modulation rates and corresponding aspects are, for example, discussed in U.S. Pat. No. 5,340,062, issued Aug. 23, 1994. The iron core of the pickup coil is relatively long, of the order of 30 inches, and extends horizontally and transversely over the underlying rail, the long core length being utilized both for high sensitivity and to assure that the coil will at all times overlie the rail irrespective of the position of the locomotive, e.g., lateral displacement of the locomotive body relative to the rails as the train traverses a curve.
Inductors of the above described type operate quite satisfactorily in diesel locomotives in which the engines drive direct current generators that, in turn, supply current to DC traction motors. However, modern diesel locomotives employ solid state switching that has made the use of alternating current traction motors possible and eliminated the high maintenance requirements associated with the use of direct current motors. The alternating current frequency can vary from approximately 20 Hz to 300 Hz in accordance with rotor speed as dictated by the speed requirements of the train. This results in the generation of an alternating current magnetic field by the AC traction motors that did not exist in the direct current powered locomotives. Being in the same frequency range as the cab signal carrier, the AC traction motors, in effect, are a source of high level noise which is received by the horizontal core pickup coils along with the coded cab current and renders them unusable as a reliable control information sensor.